Recently, enhancement of comfort and safety for the human is demanded, and in particular the method of controlling individually the environments of a limited space in which a person exists is capable of obtaining high effects in the aspects of comfort and safety, and development of various devices and techniques has been promoted. In addition, if the device for controlling the environments of a limited space can be operated only while a person is present in the space, it is effective for saving the energy and reducing consumption of the device, and hence it is attempted to detect the presence of man in a space in various methods.
A conventional example of this kind of human body detector for seat is described. For example, DE. PAT. No. 4237072, using a film type pressure sensor 1 as shown in FIG. 1, discloses a constitution of a human body detector for seat by disposing it beneath a surface cloth of a seat. The film type pressure sensor 1 is formed by disposing plural pressure sensitive resistant elements 3 mutually connected parallel electrically by a conductor 2 on a base film, and the conductor 2 is connected to two terminals at an end 4, and by measuring the resistance value between the terminals, the pressure applied to the film type pressure sensor 1 can be detected. That is, according to this constitution, when a human body is seated on the seat, the pressure by the body weight is applied to any one of the plural pressure sensitive resistant elements 3, and the resistance value of the pressure sensitive resistant element 3 loaded with pressure is lowered, and the resistance value between the two terminals at the end 4 showing the entire resistance value is lowered, and by detecting this drop of resistance value, the presence of the human body on the seat is detected.
In such constitution, however, if an object is put on the seat, the resistance value of the pressure sensitive resistant element 3 is lowered by the weight of the object, and a wrong detection may result in. Or, when the film type pressure sensor 1 is installed at a position remote from the surface cloth of the seat, the pressure applied by the human body is dispersed as departing from the surface cloth, and it is required to raise the sensitivity of the pressure sensitive resistant element 3, and if installed closely to the surface cloth in order to avoid this, the existence of itself is felt by the human body, and the comfort of sitting is impaired, and hence it was difficult to solve the contradictory problems of sensitivity and comfort. Still more, when installing the film type pressure sensor 1 in the seat, if a pressure is applied to a part of the pressure sensitive resistant element 3 due to deviation from specified position or other reason, the resistance of the pressure sensitive resistant element 3 is lowered and it may be judged that the human body is always present, and to avoid this, very careful installation on the seat is required.
Incidentally, in U.S. Pat. No. 5,404,128, as shown in FIG. 2, using vibration detecting means 5, by installing it in the seat, converting the vibration due to move of human body or breathing into an electric signal, and processing this signal in signal processing means 6, it is judged whether a human body is present or not on the seat. Herein, as vibration detecting means 5, a film type piezoelectric element is used, and by detecting fine vibrations due to respiration, heart beat or other physiological phenomenon of the human body by it, it is possible to distinguish from the case in which other object not making such vibration is placed, so that the presence of human body on the seat is securely judged. In this prior art, this human body detector is installed in the seat of a passenger car, and when installed in such vibrating environments, when a person is seated on the seat, together with the vibration of the excited vibration frequency, the vibration component of the natural frequency of the human body appears largely, and this vibration component does not appear or the level is low if appearing in the case an object is placed on the seat or there is nothing, so that the difference is evident between the presence and absence of human body. Therefore, providing the signal processing means 6 with two filters 7a, 7b, the filter characteristics are set so that the first filter 7a filters the signal component attributable to the natural vibration of the human body, and that the second filter 7b filters the signal component due to vibration of the vehicle, and, using these outputs, judging means 10 takes out and judges the vibration component of the human body from the vibration of the vehicle, so that only the presence of human body sitting on the seat can be securely judged. Herein, meanwhile, the outputs of the first and second filters 7a, 7b are respectively amplified by the amplifying means 8a, 8b, and smoothed by smoothing means 9a, 9b, and issued to judging means 10, while the information of presence or absence of human body in the seat judged by the judging means 10 is issued to control means 11 of automobile, and the control means 11 controls traveling means 12, seat control means 13, or alarm means 14 on the basis of the information of presence or absence of human body.
In such case, however, the contradictory problems of sensitivity and comfort of sitting of the vibration detecting means 5 have not been solved yet. In the two foregoing embodiments, incidentally, when the film type pressure sensor 1 or vibration detecting means 5 is installed near the surface cloth, if water is spilled or lit cigarette is dropped on the seat by mistake, the film type pressure sensor 1 or vibration detecting means 5 is easily affected by water or heat, and specified performance cannot be exhibited.